Abstract classes and methods in Dart

main.dart

// Abstract classes and methods in Dart (as of v2.5)

// See live at https://dartpad.dartlang.org/847473dc3db9ba5ad9848ecf7b00ba7e

void main() {
  
//   Expect Error! - Can't instantiate abstract classes
//   final Person p = Person();
  
  final Person sartre = Philosopher();
  print('${sartre.name} of ${sartre.residence} says, "${sartre.sayCatchPhrase()}"');
  sartre.sayCatchPhrase();
  sartre.sleep();
}

abstract class Person {
  String name;
  String residence;
  
  String sayCatchPhrase();
  
  // Default methods are allowed in abstract classes
  void sleep() {
    print('zzzzz....zzzz...zz...zzzzz.....zzzz');
  }
}

class Philosopher extends Person {
  String name = 'Sartre';
  String residence = 'France';
  
//   Expect Error! - Abstract methods can only exist inside abstract classes.
//   void abstractMethodInConcreteClass();
 
  // Must override abstract methods in first implementing concrete class.
  String sayCatchPhrase() {
    return 'Hell is other people';
  }
}